Two
phase power, like three phase, gives constant power
transfer to a linear load. But in a three wire system
it has a neutral current which is greater than the
phase currents. Also motors aren't entirely linear and
this means that despite the theory, motors running on
three phase tend to run smoother than those on two
phase. The generators at Niagara Falls installed in
1895 were the largest generators in the world at the
time and were 2 phase machines. True two-phase power
distribution is essentially obsolete. Special purpose
systems may use a 2 phase system for control.
Two-phase power may be obtained from a three-phase
system using an arrangement of transformers called a
Scott T. Today 2 phase power is used for
stepper motors, special AMD computer CPUs and a few
other specialized applications.

Modern Two-Phase Motors
(2-Phase Electric Motors)

Original
2 phase to three-phase transformers installed
at Niagara Falls in 1895 (photo courtesy Hall of
Electrical History at the Schenectady Museum,
Schenectady, New York).

This
article describes the first "polyphase"
(more than one phase) system developed for the
distribution of alternating current (ac) power. This
two-phase system was subsequently rendered obsolete,
however, by the superior three-phase system that is
now universally used throughout the world.

The
Origins of Two-Phase Power

By
Thomas J. Blalock

Today,
the large-scale generation, transmission, and
distribution of electric power is by means of the 3 phase
ac system; that is, three individual
single-phase voltages and currents having a 120°
phase relationship to each other and intermingled on
three wires (excluding a neutral). The three-phase
system has been adopted because it provides for a
constant rather than pulsating power flow to motors,
and because it is an efficient system as far as the
amount of copper required per kilowatt transmitted.
The theoretical complexity of the 3 phase system,
however, delayed its complete acceptance in the early
days of electric power system development.

During
the early 1890s, understanding the behavior of simple
single-phase ac was enough of a challenge. It was not
until Charles P. Steinmetz, the legendary General
Electric scientist, developed the concept of the use
of the "j" operator (unity magnitude at a 90°
phase angle) and complex numbers for ac circuit
calculations that the behavior of voltages and
currents in ac circuits and machines was truly
understandable. Likewise, it was not until the
introduction of what eventually came to be known as
"symmetrical components," during the early
20th century, that the calculation of three-phase
voltages and currents became relatively
straightforward. This technique utilized an
"a" operator that was of unity magnitude at
a 120° phase angle (–0.5 + j0.866). This operator
was of significant value since, in a balanced 3 phase system, the voltages and currents are at
120° phase relationships to each other.

Symmetrical
components actually facilitated calculations in
unbalanced 3 phase circuits. They were originally
known as "Fortescue components" since the
method was introduced in 1918 by Charles L. Fortescue
of the Westinghouse Electric Corporation. Significant
additional work in this area was later contributed by
Edith L. Clarke of the General Electric Company.
During the late 19th century, however, this
calculation tool did not exist, and the fact that
changes in voltage or current magnitudes in one phase
of a three-phase system affected the voltages and
currents in the other two phases contributed to the
difficulty in understanding 3 phase circuits.

Thus,
the first ventures into the realm of polyphase
(multiple phases) electric power used only two alternating current
phases rather than three. The two phases were
generated with a 90° phase difference between them,
and the system that resulted was called 2 phase power. In fact, the first two-phase generators
employed during the early 1890s were merely two
single-phase machines coupled together with their
rotors carefully set relative to each other so as to
achieve the required quadrature phase relationship.
Each generator, then, really fed a separate two-wire,
single-phase circuit. Since the two phases were
completely electrically isolated from each other,
there were no interactions between voltage and current
magnitudes in one phase with those quantities in the
other phase. Therefore, from a theoretical standpoint,
the 2 phase system was more easily understood than
was the three-phase system.

The
two phases were used together in a four-wire system to
enable the operation of the new Tesla (or induction)
motor that had been developed by Nikola Tesla. In
order to be self-starting, the Tesla motor required
some form of rotating magnetic field that had to be
produced by a polyphase type of supply. The two-phase
system was adequate for this purpose. The Westinghouse
Electric Corporation supplied the power plant and
lighting for the Colombian Exposition in Chicago in
1893. 2 phase power, produced by pairs of coupled
single-phase generators, was used throughout this
installation.

2 phase
Power at Niagara Falls

The
experience gained with the use of two-phase power at
the Colombian Exposition may have had some influence
on the decision by Westinghouse to employ a 2 phase generator design for the first ac powerhouse at
Niagara Falls, which went into operation in 1895. The
generators used at Niagara Falls were of a more
conventional design, being single machines having two
interleaved windings rather than two distinct machines
coupled together. These generators operated at a
frequency of 25 cycles (25 Hz) since it was expected
that a significant portion of the power produced would
be used to operate rotary converters so as to obtain
direct current (dc) for industrial uses such as
aluminum production. These early rotary converters
required a low frequency for satisfactory operation.

There
was obviously still a mistrust of the practicality of 3 phase
power throughout the electric power
industry at that time. For example, according to an
1896 article titled "Present Status of the
Transmission and Distribution of Electrical
Energy" in the AIEE Transactions:

Where a two-phase
transmission with separate circuits is used, then if
the separate circuits are wound on different
armatures, each can be regulated to give a constant
voltage at the receiving end. This is the case, for
instance, in the large dynamos built by the
Westinghouse Company for use at the World's Fair in
Chicago. The difficulty due to the uneven loading of
the circuits is specially marked in the case of the
three-phase system, and it is one of the principal
objections that have been urged against the employment
of this system for purposes of distribution.

It
had already been realized, however, that the 3 phase configuration was superior for
transmission from the point of view of efficiency.
Thus, special phase-changing transformers were
designed by Charles F. Scott of Westinghouse in order
to step up the 2 phase generated voltage at Niagara
Falls to 11,000-V, three-phase for transmission to
Buffalo, New York. The General Electric Company was
awarded the contract to build the phase-changing
transformers and so was licensed by Westinghouse to
utilize the connection developed by Scott for this
purpose.

At
Buffalo, some of this 3 phase power was used
for
rotary converters that supplied 110/220-V dc
power for
the Edison distribution system downtown.
However, some
of the received power was converted back into
two-phase power for general lighting purposes
in
outlying areas. Motor- generator sets were
used for
this latter conversion because the frequency
of the ac
power was increased as well in order to avoid
undesirable flickering of incandescent lamps.
The
frequency used was actually 62.5 cycles,
rather than
60 cycles, so as to simplify the design of
these
frequency changers. The conversion back to 2
phase power was motivated by the conviction, at that time,
that satisfactory voltage regulation was more
easily
achieved in the two separate phases of a
two-phase
system than in a 3 phase system.

This
belief in the superiority of 2 phase systems with
respect to voltage regulation led to the extended use
of two-phase distribution in many locales. For
example, in Cohoes, New York, (north of Albany) a 1915
hydroelectric station was designed to generate
three-phase power. However, some of that power was
converted to 2 phase using "Scott" type
transformers in order to supply an extensive network
of existing two-phase feeders for lighting, rather
than change those feeders to 3 phase operation.

William
Stanley Adopts 2 phase

William
Stanley, the man credited with the first practical
application of the ac system using transformers (in
Great Barrington, Massachusetts, in 1886),
subsequently formed the Stanley Electric Manufacturing
Company in Pittsfield, Massachusetts, in 1891. Stanley
adhered to the design and construction of two-phase
generators and motors throughout the 1890s. This was
only partly a result of his belief in the superiority
of the 2 phase system for voltage regulation
purposes. Another factor had to do with the increasing
development of three-phase equipment by his major
competitors, General Electric and Westinghouse, during
the 1890s. Stanley's decision to manufacture two-phase
equipment allowed him to avoid excessive patent
infringement problems with his competitors. Regardless
of the reasons, however, Stanley contributed to the
perpetuation of the use of two-phase power in many
locations.

The
Stanley Works itself generated and utilized 2
phase power. In 1907, this plant became the Pittsfield Works
of the General Electric Company, and the
two-phase
power system that it had inherited from
Stanley
remained in use until the closing of the
facility in
1987. In fact, to this day, there is still one
elevator in an old office building there
operating
with a 2 phase motor.

William
Stanley's company
specialized in two-phase
equipment.

The
2 phase system in this plant was somewhat unusual in
that it was a three-wire system. One wire from each
phase was combined into what was called a
"common" wire (not a "neutral").
The advantage in this was the ability to use more
commonly available three-pole circuit breakers and
switches. A disadvantage, however, was that even with
the two phases balanced, the common wire carried 1.414
times the current in the other two phase wires. Thus,
economy in pulling circuits through conduits required
the use of two different sized cables. Eventually, the
plant had two power distribution systems, the original
two-phase system and a newer 3 phase system. The
two systems were interconnected by means of
phase-changing transformers. These were of a design by
Louis F. Blume of the General Electric Company and
utilized a winding configuration differing from the
"Scott" connection, presumably to avoid
patent conflicts with the Westinghouse Electric
Corporation.

Since
Stanley supplied equipment to the local municipal
power company, the Pittsfield Electric Company,
downtown Pittsfield was also served by a 2 phase system. This, however, was the more conventional
four-wire type of two-phase distribution requiring
four-pole service switches. This 2 phase distribution system remained in use until the middle
of the last century, and vestiges of it in the form of
four-pole switches could still be found on the service
switchboard of at least one old building in Pittsfield
in the early 1980s. Also, two-phase motors were still
being used to drive the elevator motor-generator sets
in Pittsfield's only department store when it closed
in 1988.

Other
2 phase Installations

In
the village of Middle Falls, New York, (northeast of
Albany) the Niagara Mohawk Power Corporation operated
a 1900 vintage, 350-kW Stanley two-phase generator in
a small hydroelectric power station there until 1987.
Another identical unit had been retired in 1976. The

output
of the station was coupled to Niagara Mohawk's 3 phase
grid by means of phase-changing
transformers.

The
generation of 2 phase power was not exclusively an
East Coast phenomenon, however. In 1898, the Pacific
Light and Power Company installed four 300-kW
Westinghouse two-phase generators in a hydroelectric
station located in San Gabriel Canyon, near Los
Angeles, California. This station served the nearby
town of Azusa.

As
the use of ac motors expanded during the early 20th
century, the problem of providing both l15 V for
lighting and 230 V for motor use from 2 phase distribution systems became significant. One solution
was the adoption of a two-phase, five-wire system in
which center taps on both phases were connected
together to create a neutral. This, then, resulted in
a "star" configuration (analogous to the
three-phase "wye" connection) and,
technically, was a four-phase system. As such, 115 V
(single-phase) for lighting was available from any of
the four phase wires to the neutral, while 230 V (2 phase) was available for motors from the four
phase wires themselves.

In
New York City, the Bronx District of the New York
Edison Company adopted this form of secondary
distribution around 1925. At that time, the Company
was interested in upgrading its existing 2,400-V,
two-phase primary distribution system to 13,200 V, 3 phase. The connected
2 phase motor load,
however, was too great to consider changing the
secondary distribution system from two-phase to
three-phase as well, so "T"-connected
(Scott) phase-changing transformer banks were
installed to supply a 2 phase, five-wire secondary
distribution system.

During
this era, the use of the 3 phase, four-wire wye-connected
distribution system was often considered to be
unacceptable because of the nonstandard voltage (199
V) between phases with 115 V available from phase to
neutral. Early induction motors, designed for
operation at 230 V, were less satisfactory when
operated on lower voltages than are induction motors
of today. The ability of the two-phase, five-wire
distribution system to supply the standard voltages of
115/230 V was a main feature in a lengthy article
published in the AIEE Transactions in 1925 by an
engineer associated with the Philadelphia Electric
Company in Pennsylvania. This article justified the

continued
use of that system.

The
Demise of 2 phase Systems

Eventually,
a hybrid type of 3 phase
distribution system,
which was known as a three-phase, four-wire,
"delta" system, came into use in certain
regions of the United States. This system included a
center tap on one phase of a bank of delta-connected
transformers supplying 230 V. The center tap formed a
neutral and, in conjunction with the two phase wires
of that particular phase, was used to supply 115/230 V
services on a single-phase, three-wire basis. Motors
operating at 230 V were supplied from the three phase
wires of this type of service connection.

A
two-phase, four-pole service
switch in a building in Pittsfield,
Massachusetts (Tom Blalock photo).

Buildings
requiring both motor and lighting service were
sometimes provided with two separate services, a
single-phase, three-wire service for lighting and a 3 phase, three-wire service for motors. Otherwise,
a single four-wire service was brought into a
building, but care had to be exercised by electricians
so as not to use the odd phase wire along with the
neutral to supply lighting loads. This odd phase was
referred to as the "high phase" or
"wild phase" because considerably more than
115 V existed between it and the neutral. This
complication associated with the four-wire delta type
of service led to its gradual abandonment during the
latter 20th century because fewer and fewer practicing
electricians were able to truly understand it. Also,
by that time, induction motors had been developed that
operated satisfactorily on voltages lower than 230 V.
As a result, the 3 phase, wye-connected service,
giving 208 V between phases and 120 V from phase to
neutral, has become the standard commercial type of
service. Also, over the years, old 2 phase primary
distribution systems were gradually replaced with
three-phase systems. A common practice became the
conversion of a 2,300-V, two-phase, four-wire
distribution system into a 4,000/2,300-V 3 phase,
four-wire system (with neutral).

Several
clever and complex plans were devised for the
temporary supply of remaining 2 phase loads from a
new three-phase system, without the expense of
purchasing special phase-changing transformers. One
such technique took advantage of the fact that there
is a 90° phase relationship between one
phase-to-phase voltage and the voltage from the third
phase to neutral in a 3 phase, four-wire system.
Customers were encouraged to purchase three-phase
motors, rather than add to their existing inventory of
two-phase motors. Many of the old motors, however,
lasted for quite some time. Occasionally, a customer
actually had to be supplied with two services, one 2 phase
and one 3 phase.

With
rare exception today, the two-phase distribution
system has become a thing of the past. Its extensive
use throughout the 20th century, however, created
interesting situations for electrical engineers
accustomed to three-phase systems. Occasional
oversights, resulting from the unrecognized need for
four-pole motor control contactors due to the
existence of an old 2 phase system, have been known
to cause havoc for electrical equipment designers and
suppliers

In Germany and Switzerland,
where 3 phase
power was originated and developed, it is
known as Drehstrom, "rotating current" for
this property of constant power.
Ordinary AC is
called Wechselstrom, or "change current."
Nikola Tesla, the discoverer of polyphase
currents and
inventor of the induction motor, employed 2
phase current, where the phase difference is 90°. This also
can be used to create a rotating magnetic
field, and
is more efficient than single-phase, but is
not quite
as advantageous as three-phase. 2 phase
power
was once rather common in the United States,
where
Tesla was important in the introduction of AC,
but has
now gone completely out of use.

Two-phase can be supplied over three
wires, but there is no true neutral, since the phases
are not symmetrical. However, it is always easy to
double the number of phases in a transformer secondary
by making two secondary windings and connecting them
in opposing phases. Four-phase does have a neutral,
like 3 phase, but requires four wires. In fact,
three-phase is more economical than any other number
of phases. For applications like rectifiers and
synchronous converters where DC is produced, it is
most efficient to use six-phase AC input, which is
easily produced from 3 phase in a transformer.

Other
Types of 2 Phase Power SystemsMonocyclic
power was a name for an asymmetrical modified 2 phase power system used by General Electric around
1897 (championed by Charles Proteus Steinmetz and
Elihu Thomson; this usage was reportedly undertaken to
avoid patent legalities). In this system, a generator
was wound with a full-voltage single phase winding
intended for lighting loads, and with a small (usually
1/4 of the line voltage) winding which produced a
voltage in quadrature with the main windings. The
intention was to use this "power wire"
additional winding to provide starting torque for
induction motors, with the main winding providing
power for lighting loads. After the expiration of the
Westinghouse patents on symmetrical two-phase and
three-phase power distribution systems, the monocyclic
system fell out of use.

High
phase order systems for power transmission have been
built and tested. Such transmission lines use 6 or 12
phases and design practices characteristic of
extra-high voltage transmission lines. High-phase
order transmission lines may allow transfer of more
power through a given transmission line right-of-way
without the expense of a HVDC converter at each end of
the line.

Conversion
to 2 Phase Power SystemsProvided two
voltage waveforms have at least some relative
displacement on the time axis, other than a multiple
of a half-cycle, any other polyphase set of voltages
can be obtained by an array of passive transformers.
Such arrays will evenly balance the polyphase load
between the phases of the source system. For example,
balanced 2 phase power can be obtained from a 3 phase network by using two specially constructed
transformers, with taps at 50% and 86.6% of the
primary voltage. This Scott T connection produces a
true two-phase system with 90° time difference
between the phases. Another example is the generation
of higher-phase-order systems for large rectifier
systems, to produce a smoother DC output and to reduce
the harmonic currents in the supply.

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Smooth
3 Phase Power Wave Form(center)Because of its smooth wave length properties, 3 phase
electrical power is used across the U.S. and throughout the world.